A leaf area index (hereafter “LAI”) represents a total area of leaves overlapping in a direction (e.g. a vertical direction) per unit area in, for example, a plant community in agricultural land or forests. The LAI is used as one index for understanding plant growth or cultivation states.
As an LAI measurement method, a method of indirectly measuring the LAI using an illuminometer is proposed as an example. In this method, for instance, illuminance is measured above and below in a plant community, and the LAI is estimated based on the measured illuminance.
As a technique related to this, for instance, an LAI indirect measurement method is described in Patent Literature (PTL) 1.
In the method described in PTL 1, an indirect measurement system captures an image of a predetermined area for each of near-infrared light and red light, using a wide angle lens and an electronic imaging element. Next, the indirect measurement system obtains a luminance value for each of near-infrared light and red light, in each subdivision area formed by dividing the predetermined area. The indirect measurement system then calculates a luminance value ratio of near-infrared light and red light for each subdivision area, estimates a relative amount of solar radiation based on the luminance value ratio, and calculates the LAI from the relative amount of solar radiation.